The present invention relates to a pattern inspection method and apparatus which inspect a pattern formed on a green sheet, film carrier, or the like, and an alignment method which aligns a master pattern and a pattern to be measured.
PGA (Pin Grid Array) has conventionally been known as a packaging technique which meets a demand for multipin ICs and LSIs. The PGA adopts a ceramic substrate as the base of a package for attaching a chip, and wiring lines are formed to lead wire extraction positions. Formation of the ceramic substrate uses a so-called green sheet prepared by kneading an alumina powder with a liquid binder into a sheet. A paste containing a refractory metal is screen-printed onto the green sheet. The sheet is baked to sinter the green sheet and metalize the paste, which is called co-firing.
Another packaging technique is TAB (Tape Automated Bonding). According to TAB, a copper foil pattern formed on a polyimide tape carrier (TAB tape) is bonded to the electrodes of an IC chip to form external leads. The copper foil pattern is formed by adhering a copper foil to a tape carrier with an adhesive and etching the foil.
Such green sheet or tape carrier undergoes visual pattern inspection by the operator with a microscope after pattern formation. However, visual inspection of a fine pattern requires a skilled operator and hard use of the eye. As an alternative to visual inspection, there has been proposed a pattern inspection method of sensing a pattern formed on a tape carrier or the like with a TV camera and automatically inspecting the pattern (see, e.g., Japanese Patent Laid Open No. 7-110863). According to this pattern inspection method, the continuous tone image of a pattern to be measured that is sensed by the camera is binarized. The binarized pattern is inspected by comparing it with a master pattern serving as a reference.
Continuous tone image data of the pattern to be measured includes a pattern (conductor such as a copper foil pattern) and a background (base such as a green sheet bearing the conductor). In general, the conductor and base have a density difference. When a density histogram representing the density frequency of image data is created, the histogram exhibits a bimodal characteristic having two maximum values: a frequency corresponding to the base and a frequency corresponding to the conductor. To binarize continuous tone image data, the threshold is set to a valley point between the two crests.
Continuous tone image data of a pattern to be measured contains density variations due to the inclination of the work surface with respect to the camera and a change over time in illumination light quantity. If a continuous tone image containing density variations is binarized at a predetermined threshold, a defect such as a conductor deficit or disconnection which should be converted into a value “0” representing the base is converted into a value “1” representing the conductor, or a defect such as a projection or short circuit which should be converted into the value “1” representing the conductor is converted into the value “0” representing the base. When the density greatly varies, a part of the base that should be converted into “0” is converted into “1”, or a part of the conductor that should be converted into “1” is converted into “0”.
When continuous tone image data in which a conductor α is free from any defect and a conductor β has a deficit C is binarized at a threshold SH1, as shown in FIG. 18A, the conductor β becomes thin by the deficit C, as shown in FIG. 18B. The deficit C can be detected by comparing the binarization result of FIG. 18B with a master pattern.
However, if large density variations exist near the conductor β and the density becomes higher than that at the position of the conductor α (FIG. 18C), the deficit C of the conductor β cannot be detected because the deficit C which should be converted into “0” is converted into “1”, as shown in FIG. 18D, even if continuous tone image data is binarized at the threshold SH1. If density variations in FIG. 18C are very large (FIG. 18E), the base and deficit C which should be converted into “0” are converted into “1”, as shown in FIG. 18F.
When continuous tone image data (FIG. 19A) in which the conductor β has a projection K is binarized at the threshold SH1, the conductor β becomes thick by the projection K, as shown in FIG. 19B. The projection K can be detected by comparing the binarization result of FIG. 19B with a master pattern.
However, if large density variations exist near the conductor β and the density becomes lower than that at the position of the conductor α (FIG. 19C), the projection K of the conductor β cannot be detected because the projection K which should be converted into “1” is converted into “0”, as shown in FIG. 19D, even if continuous tone image data is binarized at the threshold SH1. If density variations in FIG. 19C are very large (FIG. 19E), the conductor and projection K which should be converted into “1” are converted into “0”, as shown in FIG. 19F.
In order to eliminate the influence of density variations and the like present in continuous tone image data of a pattern to be measured, a technique of optimizing the threshold has been proposed (see, e.g., Japanese Patent Laid-Open No. 2-162205 and Japanese Patent Laid-Open No. 5-248836).
A pattern inspection apparatus disclosed in Japanese Patent Laid-Open No. 2-162205 extracts the density value of a base from the continuous tone image of a pattern to be measured. A level shifted from the density value of the base by a predetermined value is defined as a binarization threshold. A pattern inspection apparatus disclosed in Japanese Patent Laid-Open No. 5-248836 samples and holds a density value when the continuous tone image of a pattern to be measured changes from the base level to the conductor level. A value calculated by adding a predetermined offset to the sampled/held value is defined as a threshold.
However, the pattern inspection apparatuses disclosed in Japanese Patent Laid-Open No. 2-162205 and Japanese Patent Laid-Open No. 5-248836 suffer a complicated optical system. This is because these pattern inspection apparatuses adopt an optical detection means in addition to an image sensing means for sensing a pattern to be measured and outputting continuous tone image data. A two-dimensional pattern signal output from the optical detection means is binarized at a fixed threshold, and the density value of the base is extracted from the continuous tone image data by using the binarized two-dimensional pattern signal as a gate signal. Also, the circuits of the pattern inspection apparatus are complicated because these apparatuses require a threshold setting circuit and binarization circuit for binarizing a two-dimensional pattern signal, in addition to a threshold setting circuit and binarization circuit for binarizing continuous tone image data.
According to a conventional threshold setting method, a defect such as the projection or deficit of a pattern to be measured may be converted into an erroneous value, and a defect of the pattern to be measured may be missed. As shown in FIG. 20, the density value of a deficit portion or disconnection portion is higher than that of the base and close to that of the conductor. To the contrary, the density value of a projection portion or short circuit portion is lower than that of the conductor and close to that of the base. For this reason, binarization using a binarization threshold SH11 converts a defect such as a deficit, pinhole, or disconnection into “1”, and a defect such as a projection, scattering, or short circuit into “0”. Even if inspection is executed for the binarization result of FIG. 20, such defect cannot be detected.
There has also been proposed a pattern inspection method of setting, as shown in FIG. 21, a value between the density value of a conductor and that of a deficit or disconnection as a binarization threshold SH12, setting a value between the density value of a projection or short circuit and that of a base as a binarization threshold SH13, and comparing a master pattern and a pattern to be measured that is binarized on the basis of the binarization thresholds SH12 and SH13 (see, e.g., Japanese Patent Laid-Open No. 10-293847). When the pattern to be measured is binarized on the basis of the binarization threshold SH12, a defect such as a deficit, pinhole, or disconnection is converted into the value “0” representing the base, as shown in FIG. 21. A defect such as a deficit, pinhole, or disconnection can be accurately detected by performing inspection for the binarized pattern to be measured. When the pattern to be measured is binarized on the basis of the binarization threshold SH13, a defect such as a projection, scattering, or short circuit is converted into the value “1” representing the conductor, as shown in FIG. 21. A defect such as a projection, scattering, or short circuit can be accurately detected by performing inspection for the binarized pattern to be measured.
A conductor 12 formed on a base 11 of an inspection work actually has a trapezoidal sectional shape whose side wall is inclined, as shown in FIG. 22. A line width Wc of the conductor 12 at a height corresponding to the binarization threshold SH12 and a line width Wp of the conductor 12 at a height corresponding to the binarization threshold SH13 are different from a line width Wo of the conductor 12 at a height corresponding to the binarization threshold SH11.
The conventional pattern inspection method determines an inspection threshold on the assumption that a pattern to be measured is binarized on the basis of the binarization threshold SH11. If a pattern to be measured that has been binarized on the basis of the binarization thresholds SH12 and SH13 is inspected, the inspection is influenced by the difference between the line widths Wc and Wp and the line width Wo. A pattern to be measured that is originally nondefective may be detected as a defect, or a defective pattern may be missed. To eliminate the influence of the sectional shape of the conductor, the operator must correct the inspection threshold in consideration of the difference in line width.